Why does it say it can't judge?

Can't judge is where every check starts. It stays until you confirm the sky is clear with the sun unobstructed on the array, stamp the time, enter the module temperature and the reading, and the sun is above the horizon. A reading under the ceiling and at or above your threshold is also can't judge: a ceiling can rule a reading out, not show a system is healthy.

Why is there no default threshold or losses figure?

They are your judgement, not the model's. The low-reading threshold starts blank, and without it a low reading is not judged. Losses start blank too, and the ceiling is then labelled before losses. There is no sky-condition factor: no sourced adjustment exists for it, so the check asks you to confirm a clear sky instead.

Which clear-sky model does it use?

Haurwitz for global horizontal irradiance, Erbs to split it into beam and diffuse, and Hay and Davies to carry it onto the array, each cited to the Sandia PV Performance Modeling Collaborative or pvlib-python pages it follows. They need only the sun's position and the date, so the check works offline and needs no licensed turbidity data.

What do I do with a reading above the ceiling or below my threshold?

Above the ceiling, check the reading, its units, AC against DC and the kWp entry before anything else. Below your threshold, rule out an export limit or the inverter's AC rating capping the reading, then test the strings with the String Test Verifier.

Does the check keep my location?

No. The latitude and longitude stay in this page's memory. The device's location is requested only when you tap Use this device's location, and it is never stored, sent anywhere or put in the address. A saved result leaves the coordinates out unless you tick the box to include them.

Callout Check

Right-Now Output Spot Check

Check whether the inverter's live reading is plausible for a clear sky, against a clear-sky ceiling built offline from the sun's position, the array and your own figures, with no irradiance meter.

Sources listedReviewed September 2026Our methodology

Inputs

View result

Right now

The sky is clear and the sun is unobstructed on the array right now

Tap when you read the inverter. The time comes from this device's clock.

Site

LatitudeDecimal degrees, negative south of the equator.

LongitudeDecimal degrees, positive east.

Coordinates stay in this page. They are not stored, not sent anywhere and never put in the address. A saved result leaves them out unless you tick the box.

Include the coordinates if I save this result

Array

TiltDegrees from horizontal.

Direction the panels faceDegrees clockwise from true north.

Ground albedoThe ground's reflectance, 0 to 1. pvlib's documentation calls 0.1 to 0.4 typical for bare or vegetated ground, rising over snow or ice.

Array sizeDC, only the modules feeding the reading.

Pmax temperature coefficientFrom the module datasheet, sign as written.

Measured now

Module temperatureMeasured on the module now. Used as entered, with no offset.

Inverter readingThe live power shown right now.

Your own figures

Losses (optional)Your own allowance. Left blank, the ceiling is before losses.

Low-reading threshold (optional)Yours to set. Without it a low reading is not judged.

Live result

Edit inputs

Verdict

Can't judge

Confirm the sky is clear and the sun is unobstructed on the array right now.
Complete the entries above to build the ceiling.
Enter the measured module temperature.
Enter the inverter's reading.

To build the ceiling, add the site's latitude and longitude, the array's tilt and the direction it faces, the ground albedo, the time of the reading, the array's DC kWp and the Pmax temperature coefficient.

Key takeaways

Builds a clear-sky ceiling for the plane of array from the sun's position, the Haurwitz clear-sky model, the Erbs beam and diffuse split and Hay and Davies transposition, plus ground reflection from the albedo you enter.
Turns that into an array ceiling from your DC kWp, the datasheet Pmax coefficient and the module temperature you measure, and labels it before losses unless you enter your own.
Gives one of three verdicts: above the clear-sky ceiling, below your threshold, or can't judge. It can't judge until you confirm a clear sky with the sun unobstructed on the array.
Shows every step as text. Location is typed or taken from the device on tap, and is never stored or put in the address.

How it works

The check builds a clear-sky ceiling for this moment from formulas alone, so it runs offline and needs no licensed data, then compares the inverter's live reading with it. Every step is shown as text under the result.

Sun. The sun's elevation and azimuth come from the NOAA Global Monitoring Laboratory solar calculator equations, the same ones the Near-Shade Sun-Path Check uses, for the time stamped from this device's clock.
Clear-sky GHI. The Haurwitz model, 1098 × cos Z × exp(−0.059 ÷ cos Z) with the apparent zenith Z, as pvlib-python 0.16.1 implements it.
Beam and diffuse. The Erbs decomposition splits GHI by the clearness index, GHI over the extraterrestrial irradiance on a level surface. The extraterrestrial figure is Spencer's series as pvlib-python 0.16.1 computes it.
Plane of array. The beam is DNI times the cosine of the angle of incidence, and is zero when the sun is behind the array. Sky diffuse follows Hay and Davies, which adds a circumsolar part to the isotropic sky; under a clear sky that part is real, and leaving it out would pull the ceiling down. The ground-reflected part uses the albedo you enter.
Array. The PVWatts module equation as the Sandia PV Performance Modeling Collaborative sets it out: kWp × plane-of-array irradiance ÷ 1000 × (1 + coefficient ÷ 100 × (module temperature − 25)), with its own low-light form at or below 125 W/m². The plane-of-array figure stands in for effective irradiance with no reflection or spectral correction, and the module temperature you measure stands in for cell temperature exactly as entered, with no offset. Your losses are applied only if you enter them; otherwise the ceiling reads before losses.

Three verdicts only. Above the clear-sky ceiling means recheck the reading, its units, AC against DC and the kWp entry. Below your threshold, a percentage of the ceiling you set yourself, points to the String Test Verifier. Everything else is Can't judge: until you confirm a clear sky with the sun unobstructed on the array, while the module temperature is missing, when the sun is below the horizon, and when a reading sits under the ceiling but not below your threshold.

No. A clear-sky ceiling for one moment cannot produce a defensible yield estimate, and it is not an expected output: it is an upper bound the live reading is checked against. Weather over time, shading, soiling, the string layout and the selected equipment also matter, so use location-specific design software for production estimates. This check is not a performance ratio or PR test and is no guarantee of what a system will produce. For the rest of a low-output callout, see the low-output playbook.

Location is the latitude and longitude you type, or the device's location when you tap for it. It stays in this page, is never stored or put in the address, and is left out of a saved result unless you tick it in.

Estimates only: for guidance, not a quote or a compliance certificate. Verify against the current AS/NZS standards and a licensed design. The maths runs in your browser. Nothing is sent to Wattbench unless you sign in and deliberately save the result.

Source ledger12 sources

Best next action

Solar System Producing Less Than ExpectedDo this next

Did this help you move the job forward?

Good to know

Frequently asked

Is the ceiling a yield or an expected output?: No. A clear-sky ceiling for one moment cannot produce a defensible yield estimate, and it is not an expected output: it is an upper bound the live reading is checked against. Weather over time, shading, soiling, the string layout and the selected equipment also matter, so use location-specific design software for production estimates. This check is not a performance ratio or PR test and is no guarantee of what a system will produce.
Why does it say it can't judge?: Can't judge is where every check starts. It stays until you confirm the sky is clear with the sun unobstructed on the array, stamp the time, enter the module temperature and the reading, and the sun is above the horizon. A reading under the ceiling and at or above your threshold is also can't judge: a ceiling can rule a reading out, not show a system is healthy.
Why is there no default threshold or losses figure?: They are your judgement, not the model's. The low-reading threshold starts blank, and without it a low reading is not judged. Losses start blank too, and the ceiling is then labelled before losses. There is no sky-condition factor: no sourced adjustment exists for it, so the check asks you to confirm a clear sky instead.
Which clear-sky model does it use?: Haurwitz for global horizontal irradiance, Erbs to split it into beam and diffuse, and Hay and Davies to carry it onto the array, each cited to the Sandia PV Performance Modeling Collaborative or pvlib-python pages it follows. They need only the sun's position and the date, so the check works offline and needs no licensed turbidity data.
What do I do with a reading above the ceiling or below my threshold?: Above the ceiling, check the reading, its units, AC against DC and the kWp entry before anything else. Below your threshold, rule out an export limit or the inverter's AC rating capping the reading, then test the strings with the String Test Verifier.
Does the check keep my location?: No. The latitude and longitude stay in this page's memory. The device's location is requested only when you tap Use this device's location, and it is never stored, sent anywhere or put in the address. A saved result leaves the coordinates out unless you tick the box to include them.
Is my data stored or sent anywhere?: The calculation runs in your browser. Nothing is sent to Wattbench unless you sign in and deliberately select Save result.

Keep calculating

More tools

Field Check

String Test Verifier

Record each string's Voc, Isc and polarity, compare them with temperature-corrected expected values and with sibling strings, and keep the readings on this device.

Expected Voc · sibling spread · consistent or review per string

Site Visit Check

Near-Shade Sun-Path Check

Find when a tree, building or chimney near the array has the sun behind it, on a solstice and equinox sun-path chart, and the row spacing that is shade-free for a window you choose.

Sun-path chart · months and times behind each obstruction · row spacing shade-free for your window

Design Tool

Roof Direction & Pitch Check

Record a roof's direction and pitch, then read source-backed generation-timing guidance without an invented yield percentage.

Direction · entered pitch · generation timing